Refractory metal composite coated article

ABSTRACT

The coated article includes a refractory metal substrate having an oxidation resistant intermetallic layer formed in situ thereon; e.g., a fused refractory metal silicide layer, and a ceramic layer applied on and adhering to the intermetallic layer to provide enhanced high temperature properties and improved resistance to premature catastrophic failure in high temperature oxidizing environments where dissimilar materials are present that may destructively react with the inermetallic layer.

This is a division, of application Ser. No. 086,023, filed on Aug. 17,1987, now U.S. Pat. No. 4,889,776.

FIELD OF THE INVENTION

The invention relates to coated refractory metal articles and methodsfor their manufacture and, in particular, to coated refractory metalarticles having improved operating temperature and life capabilities aswell as improved resistance to premature catastrophic failure in hightemperature oxidizing environments.

BACKGROUND OF THE INVENTION

The very high melting points and consequent high temperature strength ofthe so called refractory metals, including but not limited to columbium,molybdenum, tantalum, rhenium and tungsten, have made them logicalcandidates for applications in high temperature environments. However,the unacceptably poor oxidation resistance of all of these metals oralloys thereof has in the past, limited their use to applications onlyin non-oxidizing environments (inert gas or vacuum atmospheres for all,reducing atmospheres for some).

A substantial number of programs, during the period from 1954-1970,aimed at the development of oxidation resisting coatings for therefractory metals, yielded some positive results, the most notable ofwhich are covered by U.S. Pat. No. 3,540,863 issued Nov. 17, 1970, toSeymour Priceman and Lawrence Sama. The fused silicide coating is formedby applying a dried slurry of powdered silicon alloy on the substrate,then heating to a temperature and for a time to melt the alloy and reactit with the substrate to form refractory metal silicides and finallycooling to ambient temperature. The fused refractory metal silicidecoatings covered by this patent have been widely accepted and haveproven successful in real service applications over the interveningyears. The principal real service applications have been for liners inthe afterburner nozzles of gas turbine engines (e.g., the Pratt &Whitney Aircraft F-100 engine) and for thrust chambers, thrustchamber-nozzle assemblies, and nozzle extensions for liquid rocketmotors. In these applications the design wall temperatures of thecomponent may range from 2200° F. to 3000° F., which is beyond thecapability of conventional metals.

Use of fused silicide coated refractory metals in gas turbine enginesplaces burdens on the silicide coating in addition to oxidationdegradation. In particular, gas turbine engines currently employ a widevariety of metals, including aluminum, titanium, steels and of coursenickel, cobalt and iron based superalloys in various engine componentsupstream of the fused silicide coated refractory metal hot sectioncomponent. In addition a very large variety of materials are also usedas coatings for wear, corrosion and erosion resistance and as abradableseals upstream of the coated refractory metal hot section component.Therefore, there is potential for many of these other materials toaccidentally or inadvertently come in contact with the coated refractorymetal component during any reasonable period of operation. Since therefractory metal component may be operating at 2200°-3000° F. and sincethe principal constituent of the protective oxidation resistant coatingis silicon, contact of any of the above materials or oxide scalesthereof with the refractory silicide coating can result in seriousdamage to the coating due to chemical or metallurgical reactiontherewith and damage to the refractory metal substrate as a result ofloss of coating protectiveness. If the metals or coating constituents ofthe upstream components come in contact with a silicide coatedrefractory metal at a surface temperature greater than 2000° F. for asufficient time, the metal may alloy or react with the silicide coatingand result in either eutectic formation and/or localized melting of thecoating, or at the very least, localized degradation of the coatingwhich may then fail prematurely.

Degradation from oxidation and chemical/metallurgical reactions ofdissimilar materials with the silicide coating will increase as enginemanufacturers attempt to increase the temperature of the hot gas flow ingas turbine engines to enhance engine thrust and/or engine efficiency.

SUMMARY OF THE INVENTION

The invention contemplates a coated refractory metal substrate orarticle having improved operating temperature and life capabilities attemperature and, where necessary, improved resistance to prematurecatastrophic failure resulting from chemical/metallurgical reactionswith other materials in a high temperature oxidizing environment.

The invention also contemplates a coated refractory metal substrate orarticle having such improved capabilities by virtue of the presence of aspecial coating on the refractory metal.

The invention further contemplates methods for applying the specialprotective coating to a refractory metal substrate or shaped articleusing modified coating application parameters and/or treatments of thesubstrate or coated article to impart the improved operationalcapabilities to the coating.

The invention further contemplates a gas turbine or other engine usinghot gas flow in which a downstream coated refractory metal hot sectioncomponent is protected from degradation or destruction as a result ofreaction with materials, such as dissimilar metals or compounds thereof,from upstream components and coatings in the engine.

The invention provides a refractory metal substrate or article having aninner oxidation resistant intermetallic layer thereon formed in situ onthe substrate by reaction of another metal with the substrate to includean oxidation resistant intermetallic compound of the refractory metal,preferably a fused silicide of the refractory metal, and an outerceramic layer adhering to the inner layer. The ceramic layer is selectedto exhibit stability in high temperature oxidizing environments,chemical/metallurgical stability at the interface with the intermetalliclayer in such high temperature environments, adequate adherency to theintermetallic layer and, where necessary, reaction barriercharacteristics for preventing chemical/metallurgical reaction betweenthe intermetallic layer and reactive materials, such as metals, thatcould degrade or destroy the intermetallic layer and/or refractory metalsubstrate by alloying or reacting therewith and/or thermal barriercharacteristics. Furthermore, the ceramic layer can be selected to haveradiation emissivity and reflection properties to suit particular hightemperature operating environments to aid in protecting the coatedarticle. Additional ceramic layers may be applied to the outer ceramiclayer to this end.

Application of the ceramic outer layer onto the intermetallic innerlayer is effected by plasma spraying or other deposition techniques suchas chemical vapor deposition. According to method aspects of theinvention, the adherence of the ceramic layer to the intermetallic layeris enhanced by imparting increased surface roughness to theintermetallic layer. In one method aspect of the invention, therefractory metal substrate is treated such as by grit blasting or otherphysical or chemical techniques to increase substrate surface roughnesswhich is imparted, in turn, to the free surface of the intermetalliclayer formed in situ on the substrate.

In another method aspect of the invention, the heating parameters atwhich the intermetallic layer is formed in situ on the substrate aremodified to increase the surface roughness of the layer by shorteningthe time the coating layer has to "wet-out" or smooth out in the moltenstate on the substrate.

Still another method aspect of the invention involves increasing thesurface roughness of the intermetallic layer by substantially increasingits thickness since surface roughness has been observed to increase withcoating thickness.

In these method aspects of the invention, surface roughness of theintermetallic layer is maintained preferably within the range of about160 to about 300 micro-inches RMS, and more preferably within the rangeof about 200 to about 300 microinches RMS.

Another method aspect of the invention involves exposing the refractorymetal substrate having the intermetallic inner layer and ceramic outerlayer thereon to a high temperature oxidizing treatment to oxidize aconstituent, such as silicon, of the intermetallic layer through theoxygen permeable ceramic layer to form an oxidation scale, such assilica, that grows into crevices, pores and interstices of the ceramiclayer at their interface to enhance the bond therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-section through a refractory metal compositeor coated article of the invention.

FIG. 2 is a schematic illustration of a gas turbine engine including therefractory metal composite component or article of the invention.

FIG. 3 is a partial cross-section through a refractory metal compositeor coated article of the invention having a multiplex coating thereon.

BEST MODE FOR PRACTICING THE INVENTION

The invention provides a refractory metal composite article thatincludes a refractory metal substrate 2, an oxidation resistantintermetallic layer 4 on the substrate and a ceramic layer 6 adhering onthe intermetallic layer 4, FIG. 1.

In FIG. 1, opposite surfaces 2a,2b of the substrate are shown having theintermetallic layer 4, and ceramic layer 6 thereon. Of course, theinvention envisions and includes a refractory metal substrate having thelayers 4,6 on only one side or on only a portion of surfaces 2a,2b asrequired for the particular use involved.

Typically, the refractory metal substrate 2 is fabricated such as bymachining, spinning, forming, welding and other known forming techniquesto a final shape adapted for use in a particular service applicationprior to application of layers 4,6 thereon, although the invention isnot to be construed as so limited.

As used herein including the claims, the term refractory metal shallgenerally include Rows 5 and 6 of the Group IV-VII metals of thePeriodic Table of Elements (Zr,Nb,Mo,Tc,Hf,Ta,W,Re) as well as alloysthereof one with another and/or with other non-refractory elements.Columbium, molybdenum, tantalum, rhenium and tungsten as well as alloysthereof one with another and/or with other non-refractory elements wherethe refractory metal comprises at least about 50 percent by weight oftotal alloy weight are preferred refractory metals to which theinvention is especially applicable.

Columbium and alloys of columbium are expected to be the most widelyused substrate materials with respect to service applications in gasturbine engines and rocket motors where high temperature gas flow isemployed as a propulsion medium. Suitable columbium alloys, useful inthis invention are set forth in aforementioned Priceman and Sama PatentNo. 3,540,863, the teachings of which are incorporated herein byreference. Of course, the invention is not limited to use with columbiumor its alloys.

In one embodiment of the invention especially preferred for use withcolumbium alloy substrates, the intermetallic layer 4 is a silicidelayer of the type described in the aforementioned U.S. Pat. No.3,540,863, and formed by the slurry and fusing techniques described inthat patent, the teachings of which are incorporated herein byreference.

Although silicide layers of the refractory substrate metal arepreferred, other oxidation resistant intermetallic layers formed in situon the substrate by reaction of another metal therewith are possible;e.g., refractory metal aluminides, beryllides and others are consideredwithin the scope of the invention.

In forming the silicide layer 4, a slurry of powdered silicon-bearingalloy is applied to surface 2a and/or 2b of the refractory metalsubstrate 2 and then the coated substrate is heated to a temperature andfor a time sufficient to melt the powdered metal into a molten layerthat wets and reacts in situ with the substrate to form oxidationresistant intermetallic layer 4. The coated substrate is then cooled toambient temperature. A preferred powdered metal composition for use inthe slurry nominally includes by weight 60% silicon, 20% chromium and20% iron. When the slurry includes such powdered silicon alloy appliedonto a columbium alloy substrate, heating of the coated substrate is ata temperature and time sufficient to melt the powdered silicon alloy andcause it to wet and react with the columbium alloy substrate in situthereon to form a fused layer of oxidation resistant columbium silicidesin various stoichiometric ratios of columbium to silicon throughsubstantially the entire thickness of layer 4 as explained in theaforementioned U.S. Pat. No. 3,540,863 and a technical article entitled"Reliable, Practical, Protective Coatings for Refractory Metals FormedBy The Fusion Of Silicon Alloy Slurries" published in ElectrochemicalTechnology, Vol. 6, No. 9-10, Sept.-Oct. 1968 authored by Priceman andSama, the teachings of which technical article are also incorporatedherein by reference.

As will be explained hereinbelow, a method aspect of the inventioninvolves controlling the surface roughness of the outer surface 4a ofthe silicide layer to promote adherence of the ceramic layer to beapplied thereto. In particular, the surface roughness of the silicidelayer is increased compared to that normally obtained by the fusing heattreatment. As will be explained, this involves, in one case, modifyingthe heating parameters during the fusing treatment and, in other cases,pretreating the refractory metal substrate to increase surface roughnessof the fused silicide layer 4 or post treating the ceramic coated metalarticle.

In accordance with the invention, the fused silicide layer 4 is coveredor coated with a refractory ceramic layer 6 shown in FIG. 1. The ceramiclayer preferably is applied to surface 4a of the silicide layer by airplasma spraying; however, other suitable techniques for depositingceramics, such as chemical vapor deposition, could also be used to applythe desired ceramic layer 6.

Requirements for the ceramic overly layer 6 include stability in hightemperature oxidizing environments; i.e., the ceramic should not oxidizeor otherwise react with the environment to any significant adverseextent. Some ceramics may oxidize at elevated temperature but theoxidizing is self-limited and eventually reaches an acceptable minimallevel. As will be explained hereinbelow, the environment may includedissimilar metals that may deposit on or contact the ceramic layer. Theceramic layer should be stable in such situations; i.e., substantiallynon-reactive with such metals and oxides and other compounds thereof, atthe temperatures encountered in use. Substantial non-reactivity with theenvironment is desired. The ceramic should also exhibit stability incontact with the silicide layer at their interface in such hightemperature oxidizing environments so that the oxidation resistantproperties of the silicide layer are not significantly adversely alteredor affected. Generally, the ceramic layer and silicide layer should becompatible in such environments so that one layer does not significantlyadversely affect the desired performance of the other in theenvironment.

The ceramic layer and silicide layer must exhibit sufficient adherencytherebetween through all conditions of use to be encountered in the hightemperature oxidizing environments including expected thermal cyclingbetween high and low temperatures.

In some service applications the ceramic layer 6 should also exhibitthermal barrier characteristics so as to reduce the temperature of theintermetallic layer and refractory metal substrate. Also in some serviceapplications the ceramic layer should exhibit resistance to wear andabrasion to such an extent that it is not removed or worn away duringthe expected or desired time of service.

Ceramic materials which may be of use as ceramic layer 6 in practicingthe invention include alumina (Al₂ O₃), hafnia (HfO₂), ceria (CeO₂),magnesia (MgO), calcia stabilized zirconia (ZrO₂ --8w/o CaCO₃), yttriastabilized zirconia (ZrO₂ --8--20w/o Y₂ O₃), yttria (Y₂ O₃ magnesiumzirconate (ZrO₂ --24w/o MgO), magnesium aluminate (MgO--Al₂ O₃), calciumzirconate (ZrO₂ --31w/o CaO), and zirconium silicate (ZrO₂ --SiO₂).

Preferred ceramics among those listed include alumina, ceria,yttria-stabilized zirconia and yttria by virtue of their having passed athree part screening test. The first screening test consisted ofsubjecting ceramic coated columbium alloy specimens having similarsilicide layers between the ceramic layer and columbium alloy substrateto nine cycles of a slow cyclic oxidation test in which the cycle is 40minutes in duration and the temperature is varied in sine wave fashionfrom 800° F. to 2300° F. and back to 800° F. The ceramic coatedspecimens that satisfactorily completed the nine cycles were furthertested for additional fifty-six cycles in the same test. The additionalcycles constitute the second screening test. The third screening testconsists of exposing columbium alloy specimens having the silicide layerand ceramic layer thereon at 2500° F. in air with four cycles to room orambient temperature. These screening tests can be used to selectsuitable ceramics for use in applications involving high temperatureoxidizing environments where thermal cycling is required. Of course, thescreening tests can be tailored to different types of serviceapplications expected to be encountered. And, the screening testsreferred to above may be supplemented by additional tests which areunique and tailored to the expected environment to evaluate otherfactors of performance; e.g., to evaluate which ceramic exhibits maximumoperating temperature.

The preferred thickness for the ceramic layer will depend on itsintended function or functions in a particular operating environment.Preferably, the ceramic layer is applied in a thickness range of about 5mils to about 40 mils when the ceramic layer functions primarily as athermal barrier coating to reduce the temperature of the intermetalliclayer and refractory substrate. When the ceramic layer constitutesprimarily a chemical/metallurgical barrier, a thickness range of about 3mils to about 6 mils is preferred. If wear and/or erosion resistance isthe primary intended function, the ceramic layer will have a thicknessrange of about 3 mils to about 10 mils. To alter emissive properties ofthe silicide coated substrate, the ceramic layer may be only about 1/2mil to about 3 mils in thickness. Of course, the ceramic layer may servemultiple functions and the layer thickness is selected accordingly.

As mentioned hereinabove, the surface roughness of the fused silicidelayer 4 is controlled; i.e., increased, to promote adherence of theplasma sprayed ceramic layer thereto. Silicide coatings fused oncolumbium alloys by past techniques exhibit a natural micro roughness onsurface 4a of approximately 100-140 micro-inches RMS (root mean square).According to the invention, the bond strength or adherence of theceramic layer 6 to the silicide layer 4 is increased by providing arougher surface 4a on the silicide layer. Preferably, the surfaceroughness of the silicide layer is within the range of about 160 toabout 300 micro-inches RMS and, even more preferably, within about 200to 300 microinches RMS.

In accordance with the invention, surface roughness within the preferredranges for the silicide layer can be obtained by slowing the heatingrate at which the dried slurry of powdered silicon-rich alloy is fired.This slower heating rate causes the silicide compounds to remain moltenfor a shorter period of time and, in effect, shortens the time availablefor the molten material to "wet-out" or smooth out on the substrate 2 ascompared to fusing parameters used in the past. For example, in priorfusing techniques employed, the heat up time in the critical temperaturerange between 2000° F. and 2680° F. was 25 minutes, resulting in aheating rate of 23° F. per minute. In accordance with the invention, theheat up time in the same temperature range (between 2000° F. and 2680°F.) is set as one (1) hour to provide an approximate heating rate of 11°F./minute and the resultant surface roughness of the fused silicidelayer is approximately 160-180 micro-inches RMS. This increase insurface roughness increases the bonding or adherence of the ceramiclayer to the silicide layer.

Another technique for increasing surface roughness of the silicide layeris to grit blast (using 54 mesh alumina or silicon carbide abrasive) thesurface 2a of the refractory metal substrate 2 such as columbium alloyto yield an as-blasted or as-roughened surface roughness of about 200microinches RMS or greater. The silicide layer fused on such a roughenedsubstrate surface by conventional fusing techniques (i.e., not using themodified fusing technique of the preceding paragraph) will have asurface roughness of approximately 170-190 micro-inches RMS or greater.

Still another technique for increasing surface roughness of the silicidelayer is to increase the thickness of the silicide layer. For example,silicide layers with 6 mil thickness exhibit surface roughness of about180-200 micro-inches RMS as opposed to a surface roughness of about100-140 micro-inches RMS for a 3 mil thick silicide layer, both layersbeing fused under the same conventional fusing techniques (i.e., notusing the modified technique described above or substrate surfaceroughening also described above). Preferred thickness for the silicidelayer for this aspect of the invention is about 5 mils to about 8 mils.For the other embodiments of the invention described hereinabove,preferred thickness for layer 4 is about 3 mils to about 6 mils.

Another technique also employed in accordance with the invention topromote and enhance adherence of the ceramic layer to the silicide layerinvolves a post-treatment of the ceramic coated article. In particular,the ceramic coated article is exposed to a high temperature oxidizingenvironment; e.g., 2500° F. for 1-2 hours or more in air. Since theplasma sprayed ceramic layer 6 is somewhat permeable to air, thesilicide coating will oxidize at the silicide-ceramic interface to forma silica scale (SiO₂) that grows into crevices, pores and otherinterstices of the ceramic at and near their interface to fortify thebond between the layers.

Regardless of the technique employed to roughen the surface 4a of thesilicide layer, enhanced adherence of the ceramic layer 6 thereto isobtained for higher roughness silicide surfaces 4a, especially in therange of about 160 to about 300 micro-inches RMS. An even more preferredrange of surface roughness for layer 4 is about 200 to about 300micro-inches RMS.

The refractory metal composite article of the invention finds specialuse as a hot section component in the gas turbine engine illustratedschematically in FIG. 2. It can be seen that the gas turbine engineincludes an upstream compressor section 100, including compressor blades100a and vanes 100b, that delivers compressed heated air to a combustorsection 102 which may take various forms known to those skilled in theart where fuel and compressor discharge air are intermixed and ignited.The ignited hot gas flow from the compressor is delivered or passedthrough a turbine section 104 having turbine blades 104a and then isdischarged out the downstream end of the engine, all as is well known.The engine includes an afterburner section 106 in which additional fuelis injected into the already hot ignited gases from the turbine sectionfor thrust augmentation purposes. In gas turbine engines of the typeillustrated, silicide coated columbium alloy components have been usedas downstream hot section components; e.g., as liners 107 in theafterburner nozzle 108. In the afterburner section, the design wall orliner temperature may range from 2000° F. to 2700° F. during operationof the engine.

As is known, a variety of metals and alloys are utilized in the varioussections of the engine upstream of the afterburner section. These metalsand alloys include aluminum, titanium, steels and of course nickel,cobalt and iron based superalloys. Furthermore, a large variety ofmaterials including metal alloys are used as coatings for wear,corrosion and erosion resistance, particularly in the hot turbinesection, and as abradable seals.

As a result, there is a great potential for some of these other metalsand materials as well as oxides and other compounds thereof toaccidentally or inadvertently pass downstream in the gas flow of theengine and come in contact with the liner of the afterburner section.Since the liner made of silicide coated columbium alloy may be operatingat 2000° F. to 2700° F. and since the principal constituent of thesilicide coating is silicon, contact therewith at such high temperaturesof any of the dissimilar metals or oxides thereof or other materialsfrom upstream components can result in serious damage to the silicidedue to chemical or metallurgical reaction therewith. It is likely thatany of the metals or alloys referred to hereinabove that are commonlyused in the upstream sections of the engine, if in contact with thesilicide coating on the refractory metal liner at surface temperaturesof 2200° F. or greater for sufficient time, will alloy or reacttherewith to effect either eutectic formation or localized melting ofthe silicide, or at the very least, localized degradation of thesilicide coating which may then fail prematurely.

A refractory metal afterburner liner coated in accordance with theinvention will be quite beneficial in that the ceramic layer 6 adheringon the silicide or other intermetallic layer 4 will function as abarrier to chemical or metallurgical attack by metals or compoundsthereof that may contact the ceramic layer when the liner is attemperatures of 2200° F. or greater in the afterburner. As mentionedhereinabove, the ceramic layer 6 is selected to be stable; i.e.,substantially non-reactive, to these metals or compounds from upstreamengine components. By coating the silicide layer 4 with the ceramiclayer 6 in accordance with the invention, the potentially seriousproblem of reaction of the silicide coating with other materials ofconstruction found in upstream sections of the engine and inadvertentlybrought in contact with the liner during engine operation can beminimized or avoided altogether. The ceramic layer 6 is also beneficialin that it functions as a thermal barrier on the silicide layer 4 andsubstrate 2 to reduce the temperature thereof and, in effect, allow themaximum operating temperature of the liner to be increased. This effectallows a higher gas discharge temperature to be used in the afterburnersection or other sections of the engine. The ceramic layer 6 alsofunctions to provide enhanced wear or erosion resistance to the hot gasflow in such engine applications.

The silicide layer 4 provides oxidation protection to the substrate.This function is not adversely affected by ceramic layer 6 and, instead,ceramic layer 6 lowers the temperature of the silicide layer andsubstrate as a result of thermal barrier action and may concurrentlyalso inhibit accessibility of oxygen to the surface of the silicidelayer.

Of course, the refractory metal composite article of the invention mayalso find use as a component in other hot sections of the gas turbineengine illustrated.

As is apparent, in many service applications, the ceramic overlaycoating can perform several functions. In an application in which asignificant portion of the heat transferred to a coated refractory metalcomponent is by radiation and there is the potential for heat to beextracted from the component by active or passive cooling methods, thenit is feasible to have a ceramic layer 6 that would at the same timeserve as (1) a barrier to prevent metallurgical alloying leading tomelting, (2) as a thermal barrier coating to reduce the temperature atthe surface of the silicide coating and of the basic refractory metalsubstrate and (3) as a highly reflective coating (low emittance, lowabsorptance) to aid in minimizing the temperature of the silicidethereby lengthening its life and reducing the temperature of therefractory substrate. In this example, if it is feasible to radiationcool the component from areas thereof opposite the heat source, then aceramic coating or layer 6 in those areas could be selected that wouldserve as (1) a metallurgical barrier and (2) as a high emissivitycoating to aid in rejecting heat and reducing the part temperature withthe benefits noted above, e.g., see FIG. 3. A prime example of such aservice application is in small restartable liquid rocket motorcombustion chambers and nozzles. These are typically used as reactioncontrol motors for missiles and spacecraft including orbitingsatellites. They operate at the highest possible temperatures tomaximize the fuel efficiency. Consequently, many are made of columbiumalloys and are protected from oxidation by silicide coatings, primarilythat described in aforementioned U.S. Pat. No. 3,540,863. Usually thesenozzles are also radiation cooled. Therefore, the performance and/orlife of such devices would be improved by overlaying the silicidecoating 4 on the inner hot surfaces with a ceramic coating 6 that wouldhave a low emittance and low thermal conductivity while simultaneouslyserving as a metallurgical reaction barrier.

In the case of the small liquid rocket motor, the ceramic coating on theinside hot surface could be yttria stabilized zirconia which also has alower emittance than the silicide layer. However, the coating may be inturn overcoated with an alumina layer 14 which has a still loweremittance and has better erosion resistance, e.g., see FIG. 3 showingsuch a multiplex coating system. Both of these ceramic overlay coatingswill serve simultaneously as metallurgical barrier coatings. It isdesirable that the outer heat-radiating surface of such a radiationcooled component have a high radiation emissivity. Since the emissivityof the silicide coating in the oxidized condition is quite high, itwould seem that no additional coating would be required. However, sincethese rockets are employed in the hard vacuum of space, there is littlelikelihood for oxidation of the silicide layer on outside radiatingsurface. Therefore, the silicide coating on the outside radiatingsurfaces will probably not be oxidized in service and thus will not havea desired stable high emissivity. Consequently, it would be beneficialto apply a stable high emissivity ceramic coating directly on theoutside radiating surface of the silicide layer 4. This coating could behafnia, hafnia-titania or yttria stabilized zirconia-titania, e.g., seeFIG. 3 showing such a multiplex coating system.

While the invention has been described hereinabove with respect toapplying a fused silicide layer to the refractory metal substrate, itwill be apparent to those skilled in the art that techniques other thanfusing may be employed. For example, the silicide or other intermetalliclayer might be applied by pack cementation processes or other knownprocesses wherein the intermetallic layer is formed in situ on thesubstrate to include an oxidation resistant intermetallic compound ofthe refractory metal.

To illustrate the invention in more detail, the following example isprovided:

EXAMPLE

Four test specimens of the columbium alloy designated as Cb 752(Cb-10%W-2.5%Zr) were prepared using 0.022 inch thick sheet material.The specimens were 3"×3" squares and contained small mounting holes tofacilitate subsequent attachment to a test fixture.

The specimens were first coated with a fused silicide coating of thecomposition Si-20%Cr-20%Fe in a process utilizing the following steps:

1. Vapor degreasing parts in trichloroethylene.

2. Grit blast all surfaces with 120 mesh iron grit.

3. Make slurry consisting of powders in the proportions Si-20%Cr-20%Feby weight and add sufficient nitro-cellulose lacquer (Raffi & SwansonL-18 or equivalent) to result, when mechanically stirred, in a slurrywith a viscosity of 100-200 centipoise.

The powders all are 99%+ pure and are -325 mesh size.

4. Spray all surfaces of the specimens with the above slurry to resultin an air dried coating measuring approximately 9 mils in thickness andhaving a unit weight of approximately 40 mg/cm². Air dry 4 hours.

5. Set parts on alumina or quartz pads in electrically heated, cold wallvacuum furnace chamber. Evacuate down to 10⁻⁴ torr.

6. Raise power to furnace but hold power level whenever pressure exceeds10⁻³ torr. When temperature reaches approximately 2000° F. raise powerto effect a linear rise in temperature of approximately 11° F./Minute sothat in approximately 1 hour a temperature of about 2680° F. isachieved. Hold temperature at 2680° F. for 1 hour then turn off power toheating elements and let furnace cool. When furnace has cooled to200°-400 F. backfill with inert gas.

7. Open furnace and remove parts. Perform required non-destructivetesting and other quality checks.

Two of the above silicide coated columbium alloy test specimens weresubsequently overcoated with a ceramic thermal barrier coating on oneside as follows:

1. The parts were handled only with lint free cotton gloves afterremoval from vacuum furnace.

2. The parts were suitably fixtured in an exhaust hood to present theface to be coated directly towards the open end of the hood.

3. One specimen was coated with 0.015 inch thick and one specimen with0.020 inch thick yttria stabilized zirconia coatings (Metco 202 NS,composition 80% zirconia-20% yttria Metco Inc.) by the air plasma sprayprocess using the following parameters:

    ______________________________________                                        a.         Gun            Metco 7M                                            b.         Nozzle         Metco GH                                            c.         Powder Port    Metco #2                                            d.         Primary Gas    Type Argon                                                                    Pres. 100 PSI                                                                 Flow 80 SCFM                                        e.         Secondary Gas  Type Hydrogen                                                                 Pres. 50 PSI                                                                  Flow 15 SCFM                                        f.         Arc Amps       500                                                            Arc Volts      64-70                                               g.         Spray Distance 21/2 Inches                                         h.         Spray Rate     6#/Hour                                             ______________________________________                                    

4. Spraying is accomplished with sufficient passes and at such atraverse rate as to prevent the specimen from exceeding 500° F. duringthe spraying operation.

The effectiveness of the duplex coating was tested as follows:

1. Each duplex coated (DC) specimen was paired with a similar silicidecoated specimen without a ceramic coating thereon.

2. Each pair was separately tested by attaching each specimen to an armextending from an air cylinder actuated by a timer so that each specimenwas alternately moved from an oxyacetylene torch flame to a cold airblast every five minutes. The torch was first adjusted while on thespecimen without a ceramic coating so that a temperature of 3000° F. wasreached on the front side (flame side) as measured with an opticalpyrometer. At that point the timers were actuated. During the test thetemperatures of the front side and back side were measured and recordedon each specimen during each cycle. The tests were continued for 7 fiveminute cycles. At that point one of the base line specimens (without theceramic coating) exhibited a burn through and the other appeared to bevery close to failure. The DC specimens were completely intact. The0.015 inch thick DC specimens exhibited a backface temperature averaging104° F. below the baseline specimen tested with it as a result ofthermal barrier action of the ceramic layer. The 0.020 inch thick DCspecimen displayed a backface temperature averaging 125° F. below itscorresponding baseline specimen for the same reason.

While certain preferred embodiments of the invention have been describedin detail hereinabove, those familiar with this art will recognize thatvarious modifications and changes can be made therein for practicing theinvention within the scope of the appended claims which are intended toinclude equivalents of such embodiments.

I claim:
 1. In an engine using a gas flow past an upstream enginecomponent toward a downstream engine component operating at elevatedtemperature, the combination of said downstream component comprising asubstrate of a refractory metal, an oxidation resistant intermetalliclayer on the substrate and a ceramic layer adhering to the intermetalliclayer, said upstream component comprising a material potentiallydestructively reactive with said intermetallic layer so as to degradesame when in contact therewith at elevated temperature, said ceramiclayer providing a barrier between said intermetallic layer and saidmaterial in the event said material comes in contact with saiddownstream component so as to prevent degradation of said intermetalliclayer.
 2. The combination of claim 1 wherein the intermetallic layer isa silicon-bearing layer including an oxidation resistant silicide of therefractory metal.
 3. The combination of claim 2 wherein the upstreamcomponent includes a metal which is chemically or metallurgicallyreactive with the silicide of the refractory metal so as to degrade it.4. The combination of claim 1 or 2 wherein the refractory metal isselected from the group consisting essentially of columbium, molybdenum,tantalum, rhenium and tungsten.
 5. The combination of claim 1 whereinthe thickness of the ceramic layer on the downstream component is about3 mils to about 6 mils.
 6. The combination of claim 1 wherein theceramic layer is selected from the group consisting essentially ofalumina, ceria, zirconia and yttria.